The Moon ages 40 million years

Since the last astronauts of the Apollo program went to the Moon in 1972, no human has set foot on it again, but the samples they brought back from our satellite continue to be studied and offer new data, such as those revealed by a team of scientists this Monday.

A new analysis of the dust collected by astronauts Eugene Cernan and Harrison Schmitt during the last mission to explore the Moon, Apollo 17, has aged our satellite by 40 million years. As the authors explain in a study published in the journal Geochemical Perspectives Letters, its age would be at least 4.46 billion years.. Previous studies had established the age of the Moon at 4.425 million years.

For astrophysicists, determining when our satellite formed is one of the most interesting aspects of studying our solar system.. The most accepted theory is that when the Earth was forming, a large object the size of Mars collided with our planet.. The largest fragment that broke off during that violent collision gave rise to our Moon.

“It is important to know when the Moon formed because it is an important partner in our planetary system: it stabilizes the Earth's axis of rotation, and is the reason why the day has 24 hours and why we have tides,” he explained. Philipp Heck, curator of meteorites and polar studies at the Field Museum in Chicago and lead author of this study.

“It is amazing to be able to have evidence that the rock you have in your hands is the oldest part of the Moon that we have found so far,” said Jennika Greer, co-author of this study that arose when she was doing her doctorate in the University of Chicago. As this scientist who is currently researching at the University of Glasgow explains, these samples are “an anchor to answer so many questions about the Earth.”. “When you know how old something is, you can better understand what has happened to it throughout its history.”

Dust collected by astronauts from the Apollo 17 mission contains tiny crystals that formed billions of years ago, and scientists say these crystals reveal when the Moon must have formed.. This is because when this Mars-sized object violently collided with the Earth and formed the Moon, the enormous energy released during the collision melted the rock that ended up forming the surface of the Moon.. “When the surface melts, zircon crystals cannot form or survive. Therefore, the crystals on the Moon's surface must have formed after the lunar magma ocean cooled.. Otherwise, they would have melted and their chemical signatures would have been erased,” argues Philipp Heck.

Since the crystals must have formed after the magma ocean cooled, determining the age of the zircon crystals would reveal the minimum possible age of the Moon.. An earlier study by co-author Bidong Zhang suggested this age, but this study published Monday marks the first time an analytical method called atomic probe tomography has been used to “determine” the age of this oldest known lunar crystal.

“These samples were brought to Earth half a century ago, but it is now that we have the necessary tools to carry out microanalyses at the required level, including atom tomography,” says Dieter Isheim, researcher at Northwesern University and co-author of the study. , which considers this study “a testament to the immense technological progress we have made since 1972, when the last manned mission to the Moon returned to Earth”.

Jennika Greer with the instrument with which she analyzed lunar dust Dieter Isheim /Un. Northwestern.

Using instruments from Northwestern University, they analyzed how many atoms within the zircon crystals had undergone radioactive decay.. When an atom (made up of protons and neutrons) undergoes a disintegration process, it loses some of these protons and neutrons and transforms into other elements.. For example, uranium decomposes into lead. Scientists can find out how long it takes for this process to occur, and by determining the proportion of different uranium and lead atoms (called isotopes) present in a sample, they can find out how old it is.

The radiometric dating they have used to establish the minimum age of the Moon works a bit like an hourglass, as Heck compares: “In an hourglass, the sand flows from one glass bulb to another, and the passage of the time is indicated by the accumulation of sand in the lower bulb. Radiometric dating works in a similar way, counting the number of parent atoms and the number of daughter atoms they have transformed into.. “Then the passage of time can be calculated because the rate of transformation is known.”. The ratio of lead isotopes the researchers found indicated that the sample was approximately 4.46 billion years old, meaning the Moon therefore has to be at least that old.

Crystals in lunar dust, seen under the microscope Jennika Greer.

“These crystals are the oldest solid elements that we know of among those that formed after the great impact. And knowing the age of these crystals serves as an anchor to establish lunar chronology,” explained Heck, who highlights the importance of studying the Moon.. “It's a part of our natural system that we want to understand better, and our study provides a small piece of the puzzle in that whole picture.”. And, as he assures, “without the Moon, life on Earth would be different.”

Between 1969 and 1972, the six manned Apollo missions brought a total of 382 kilograms of rocks, pebbles, sand and dust from the lunar surface to Earth.. There were 2,200 samples collected in six different locations on our satellite. In addition, three Soviet robotic spacecraft were able to collect on the Moon and bring to Earth about 300 grams of samples collected in other places on our satellite.. The Chinese also managed to bring samples from the Moon to Earth in 2020 with their Chang'e-5 robotic spacecraft.

Most of the lunar samples brought back during the Apollo program are preserved at NASA's Johnson Space Center in Houston, and some of them have been distributed to different countries and research centers.